How do I switch the map to night mode on Edge 1040?

The 1040 does not employ a night mode on the map page.

Wow, what a fail..

Is there an alternative map that does? Or is that not a map thing? 

It is not really a map thing, but rather a map theme

I have been wanting this as well.  My Epix 2 has a map dark mode yet the Edge 1040 doesn't.  Hopefully it's on their "do-to" list for the future.  

For me it’s a great feature that the map remains in its normal ‘day’ colors when the other display elements are in night mode. For me the usability of the map in night-colors is poor, particularly if the map uses functional colors to indicate cycle track type etc (like openfietsmap)

Just my 2 cent ...
I'm not aware of many OSM alternatives with night mode, but "USERBEAM"s OSM card's do. "">https://www.userbeam.de/"
I use them partly aside OpenMTB maps "">openmtbmap.org/" which I feel are the most detailed MTB cards for Garmin and also have a set of cards for street biking. So I've different maps linked to my profiles (MTB and Street).

I remember the 830 does night mode... not sure why the 1040 couldn't do it... b'cos the screen looks pale when comparing to 830?

I never used night mode on my 530 because, to me, the maps looked much less clear, route & position difficult to see. But the odd thing is if you look at the 1040 map element color definition files in the MapThemes directory then all of them have night as well as day mode definitions so if all the necessary data is available you would have to wonder why the 1040 does not just use it, what’s the “Garmin” logic behind a decision to not support night mode?

With the current behaviour I still probably would not use night mode but might if night mode data screen colors save battery and I still get the day mode map.

If you want night colors on Edge map, you could just use a map theme that has those set as day colors. Granted, switching map themes is not quite as practical as having colors change automatically, but it's still easily doable, and on device.

I added Edge default themes' night colors as separate templates to my little theme editor here. You can download them as-is or tweak the colors first.